SSC CGL 2019 PYQ — Question 58

The correct answer is option 4 i.e. 6.

Downstream speed = Speed of boat in still water + Speed of stream

Upstream speed = Speed of boat in still water – Speed of stream

Distance covered downstream = 7.2 km

Distance covered upstream =3.2 km

Suppose,

x = speed of the boat in still water

a = speed of the stream

Hence,

The speed of boat downstream be (x + a) and upstream be (x - a)

According to the question,

⇒ ${{7.2\over x+a} + {3.2\over x-a}} = 2$.....(1)

and,

⇒ ${{1.5\over x+a} + {0.6\over x-a}} = {24\over 60}$.....(2)

Multiply by (3.2/0.6)

⇒ ${{8\over x+a} + {3.2\over x-a}} = {32\over 15}$.....(3)

Subtracting eq(1) and (3)

⇒ ${{0.8\over x+a}} = {2\over 15}$

⇒ (x + a) = 6

Speed of the boat downstream = 6 km/hr